Sarcopenia, or the age-related loss of skeletal muscle mass, affects as many as 70% of adults over age 60, and continues to progress such that the estimated skeletal muscle mass of a typical 80-year-old is roughly 60% of that of their 20s. Additionally, senile skeletal muscle displays increased stiffness which can have numerous effects on health, physical function and independence. There is, therefore, a critical need for accurate, non-invasive techniques to quantify passive skeletal muscle stiffness. The long term goal of this project is to develop effective diagnostic strategies and interventions that will improve the physical functioning and independence of older adults. The objectives of this application, progressing towards the long-term goal, are to develop and validate a non-invasive method to quantify muscle stiffness, and to non-invasively assess the stiffness of skeletal muscle in both young and older adults. The central hypothesis is that novel, non-invasive ultrasound technology will be a reliable, valid technique to evaluate the passive stiffness of skeletal muscle throughout its functional range of motion. The rationale for the proposed research is that understanding the changes in skeletal muscle passive stiffness associated with aging may provide a basis for development of effective diagnostic and treatment procedures. Two specific aims will be pursued to objectively test the hypothesis. First, a non-invasive method will be developed and validated for assessing passive skeletal muscle stiffness throughout the physiologic range of tension. Second, the muscle stiffness changes with sarcopenia in humans will be quantified. The expected outcomes of the work proposed include developing the non-invasive methodology to understand the impact of aging and sarcopenia on passive properties of skeletal muscle. This contribution will be significant because it is the firs step towards developing effective diagnostic strategies and treatment interventions that will improve the diagnosis, assessment and prevention of sarcopenia progression and limited musculoskeletal function.